There is a well-known need for measuring the number of revolutions of the crankshaft in vehicle engines. In most normal cases, measurements for this purpose could be done with the aid of some kind of sensor, such as a sensor that generates pulses of optical or inductive type mounted on e.g. the flywheel.
In some situations it may be desirable to be able to measure the rotational speed of the engine crankshaft during the course of the engine starting process. The measurement readings could then be used to establish the condition of or give a diagnosis on the starting system. By the expression "starting system" should in this case be understood the circuit comprising the starter motor, the generator, the starter battery, the ignition system and the wiring required to interconnect these items. A diagnosis of this kind is instrumental in establishing a value of the rotational speed during the starting process, and this value is then compared with an already known set value established in dependence on a number of parameters, such as the engine temperature or the state of charge of the battery. When deviations from the expected set value of the rotational speed during the starting process are found the conclusion may be drawn that some fault exists in the starting system.
If one knows the rotational speed when a certain piston is approaching its top-dead-center position, it may also be possible to decide whether or not fuel should be injected into the engine. Consequently, unnecessary fuel injections may be avoided, since an explosion could not occur in any case because of the too low rotational speed of the crankshaft.
In connection with the engine starting process the demands on accuracy of measurement of the rotional speed or number of revolutions are particularly severe, for which reason sensors of conventional kinds are not entirely satisfactory. In the course of the starting process, the number of crankshaft revolutions are few but the speed thereof varies considerably. During conditions of continuous operation the number of revolutions of the crankshaft remains comparatively stable, a condition that makes it possible to establish a mean value derived from several revolutions. This mean value is comparatively accurate since there is very little variation in the number of revolutions. On the other hand, since the rotational speed of the crankshaft thus varies heavily during the starting process, a mean value on the basis thereof would be grossly deceptive.
Conventional sensors generally are not sufficiently accurate or fail to provide sufficient resolution to be used for measuring purposes during the engine starting process. A sensor designed for this purpose therefore would be unnecessarily expensive.
A considerable advantage therefore would be gained, if it were possible to obviate the need for a sensor of a conventional kind.
By means of the subject invention it has become possible to measure the rotational speed of the crankshaft in an internal combustion engine. A measuring method of this kind is of particular use during the starting process and obviates the need for conventional revolution counters in that it makes use of the existing measurement of current or voltage associated with the vehicle starter motor.
The above purpose is achieved by means of a measuring method of the kind outlined in the introduction which is characterized in that the voltage across a measuring shunt inserted in the current circuit between the battery and the starter motor is measured on several occasions recurring essentially periodically in connection with the activation of the starter motor, in that the values of the current delivered from the starter battery to the starter motor are derived from the voltage measuring results and are stored in the memory of a measuring computer, and are used to establish the status of the combustion engine and its starting system.